In recent years, as material life improves, people have become more conscious concerning the issues of health. Most people are unaware of their current health condition or environmental condition, and overlook signals emitted by their body. Thus, in order to let the public be able to observe their own physiological condition, various measuring devices having been gradually emerging. As such, the public can use the various measuring devices to monitor their own physiological condition, and immediately determine if there are any unusual problems occurred with their health condition. By this manner, the user can pay more attention to their physical condition, and make any suitable adjustments.
Currently, regarding monitoring physiological signals for health conditions, heart sounds and lung sounds are some valuable factors for consideration. During a heart cycle, since the heart muscles contract and expand, the valves inside the heart open and close, and blood flow beats against the heart chamber wall and the aorta wall, some turbulences are formed, which causes some vibration that transmits to the surface of chest. When a stethoscope is placed at a specific position of the surface of chest, some heart sounds are heard, which may be used to determine if the heart is normal or not.
Lung sounds are also acoustic signals, which occur mainly from gas exchanging in the alveolus of the lung lobe. The oxygen in air can be absorbed by the body, and the carbon dioxide in the body can be disposed of at the same time. The lungs undergo contraction and expansion, which drives the gas flow, and then the sound is generated through the air flow vibration in the breathing passage. Through confirmation from related research, the strength of the lung sound is related to the air flow rate in the breathing passage, and the frequency distribution of the lung sound is related to the tension of the breathing passage. The frequency distribution of lung sound is very broad, ranging from 100 Hz to around 2000 Hz, and the frequency spectrum is different according to different measuring positions.
In the current technical fields, sensing heart sounds at a specific frequency range from 10 Hz to 1 kHz has a specific purpose. For example, signals sensed in a low frequency range (15-400 Hz) can be used to determine if the heart is normal or not. Through the auscultation using a stethoscope by a physician or through a microphone or a micro-electro-mechanical system (MEMS), these signals can be sensed to obtain a phonocardiogram (PCG) for the physician to observe.